In amphibia, the ear structures which transduce airborne sound appear during metamorphosis. In Rana, this new auditory projection to the brain appears at about the same time as the disappearance of lateral line afferent fibers. At this time, the dorsal medullary neurona must either experience a major change in their afferents, be replaced by migrated neurons, or give evidence of the existence of a dual population in the region. Thus, a new auditory nucleus is arising, whether from neurons previously related to the vestibular or lateral line systems or from naive neurons. The nucleus must establish the next link in the auditory pathway and thereby distinguish itself from adjacent nuclei. We wish to determine how the appearance of a sensory modality affects the central pathways and the neurons which comprise the pathways. We will identify and study the morphology of the auditory neurons in the auditory ganglia, medullary nucleus, superior olive and torus semicircularis. Morphometric analysis of perikaryal and dendritic form will be correlated with the development of auditory structures to determine the impact of afferent fiber systems upon sensory pathway neurons. Time-correlated studies of the auditory pathways will be made with HRP and autoradiographic tracing of retro- and anterograde transport.